Large areas may be covered via satellite. As the coverage areas are generally very large and the available frequency spectrum is limited, very careful frequency planning is necessitated in order to avoid interferences.
In the following, conventional concepts for frequency planning are described. Ideally, three frequencies are sufficient in frequency planning. FIG. 6 thus shows, in a graphical illustration, the principle of frequency planning with three frequencies. Here, regions are shown as hexagonal flat areas in a first approximation. Each region uses a frequency. In directly adjacent regions, other frequencies are used. In the graphical representation of FIG. 6, the used frequency is designated with a number for purposes of illustration.
Generally and/or in practice, however, the boundaries of the coverage areas are not as regular as shown in FIG. 6, so that the ideal planning shown in FIG. 6 is often not realizable. In practice, there are thus typically necessitated at least four different frequencies or even more frequencies to allow suitable frequency planning. The planning is additionally complicated when the bandwidth requirements in the individual coverage areas are different. The different bandwidth requirements may either be achieved via a scalable bandwidth, or a very large number of narrow-band carriers is used and a higher capacity in a region is achieved by associating several carrier frequencies with the region. Direct neighbors then use different frequencies. Frequency planning with seven frequencies is illustrated as an example in FIG. 7.
However, narrow-band systems with many carrier frequencies have the disadvantage that users have to choose very early which frequencies they want to decode. But modern multimedia terminals are implemented so that several services are received in parallel. A typical application scenario is the parallel reception of many and/or at least several channels. One channel is received live, for example, while the other channels are either directly stored or evaluated via filters. Corresponding to these application scenarios, it is desired that many channels are received in parallel.
In the following, the method of the segmented orthogonal frequency division multiplex (also referred to as segmented OFDM) is described. In the orthogonal frequency division multiplex (OFDM), K sub-carriers are combined to form a block. The modulation may, for example, be divided into four different parts:
1. Mapping: A group of information symbols (bits) determines the amplitude and the phase position of a sub-carrier. The number of bits used in a mapping depends on the selected constellation. In a QPSK constellation, two bits are used per carrier. In a QAM16 modulation and/or constellation, however, four bits are used per carrier.
2. Forming an OFDM symbol: K carriers are combined to form a symbol. Generally, there are additionally added L1 carriers as pilots and/or pilot tones and L2 unused carriers. Pilots are sub-carriers that carry information known to the receiver and may thus be used for synchronization and channel estimation. Unused carriers are carriers with the amplitude 0 and serve to create gaps in the spectrum, because ideally rectangular filters cannot be realized. All in all, there are thus N sub-carriers.
3. The carriers and/or sub-carriers are then transformed to a sequence of M samples using a transform (generally a fast inverse Fourier transform, also referred to as FFT-1). Here, a so-called guard interval may also be keyed in.
4. Optionally, a preamble or introduction may be added, for example to simplify the synchronization to a receiver. The above-described process is exemplarily illustrated for N=768, K=552, L1=1 and L2=215 with the following formulas. In the chosen example, there is additionally used differential coding. The pilot tone carrier thus also serves as a reference point for the differential coding.
                                          c            _                                i            ,                          k              ′                                      =                                  ⁢                  {                                                                                          c                                          i                      ,                                              k                        ′                                                                              =                  0                                                                                                                                      ⁢                                                                                                                                          for                            ⁢                                                                                                                  ⁢                                                          k                              ′                                                                                =                                                                                                                    -                                384                                                            ⁢                                                                                                                          ⁢                              …                                                        -                            277                                                                                                                                                                                        (                                                      lower                            ⁢                                                                                                                  ⁢                            guard                            ⁢                                                                                                                  ⁢                            band                                                    )                                                                                                      ⁢                                                                                                                                                                                  c                                          i                      ,                                              k                        ′                                                                              =                                                            1                                              2                                                              ⁢                                          (                                              1                        +                        j                                            )                                                                                                                                                                            ⁢                                                                                                                                          for                            ⁢                                                                                                                  ⁢                                                          k                              ′                                                                                =                                                      -                            276                                                                                                                                                                                        (                                                      reference                            ⁢                                                                                                                  ⁢                            SCS                                                    )                                                                                                      ⁢                                                                                                                                                                                  c                                          i                      ,                                              k                        ′                                                                              =                                      [                                                                                                                        1                                                          2                                                                                                                                                                                                                                      [                                                                                                                                                                                                                  (                                                                                  1                                          -                                                                                      2                                            ⁢                                                                                                                                                                                  ⁢                                                                                          b                                                                                                                                                i                                                  ·                                                  2                                                  ·                                                                                                      (                                                                                                                                                                  k                                                        ′                                                                                                            +                                                      317                                                                                                        )                                                                                                                                                  +                                                2                                                                                                                                                                                                                    )                                                                            +                                                                              j                                        ·                                                                                                                                                                                                                                                                                        (                                                                              1                                        -                                                                                  2                                          ⁢                                                                                                                                                                          ⁢                                                                                      b                                                                                                                                          i                                                ·                                                2                                                ·                                                                                                  (                                                                                                                                                            k                                                      ′                                                                                                        +                                                    317                                                                                                    )                                                                                                                                            +                                              1                                                                                                                                                                                                          )                                                                                                                                                                  ]                                                        ·                                                                                                                                                                                                          c                                                                                                i                                  ·                                                                      k                                    ′                                                                                                  -                                1                                                                                      ·                                                          ⅇ                                                                                                -                                  j                                                                ⁢                                                                  π                                  4                                                                                                                                                                                                          ]                                                                                                                                                        ⁢                                                                                                                                          for                            ⁢                                                                                                                  ⁢                                                          k                              ′                                                                                =                                                                                                                    -                                275                                                            ⁢                                                                                                                          ⁢                              …                                                        +                            276                                                                                                                                                                                        (                                                      active                            ⁢                                                                                                                  ⁢                            SCS                                                    )                                                                                                                                                                                                              c                                          i                      ,                                              k                        ′                                                                              =                  0                                                                                                                                      ⁢                                                                                                                                          for                            ⁢                                                                                                                  ⁢                                                          k                              ′                                                                                =                                                      277                            ⁢                                                                                                                  ⁢                            …                            ⁢                                                                                                                  ⁢                            383                                                                                                                                                                                        (                                                      upper                            ⁢                                                                                                                  ⁢                            guard                            ⁢                                                                                                                  ⁢                            band                                                    )                                                                                                                                                                                                      s            BSB                    ⁡                      (            t            )                          =                              ∑                          m              =              0                        ∞                    ⁢                                          ⁢                      [                                                                                                                              A                        AMSS                                            ⁢                                              g                        ⁡                                                  (                                                      t                            -                                                          mT                              TPLF                                                                                )                                                                                      +                                                                                                                                                                  ∑                                                  l                          =                          0                                                                          L                          -                          1                                                                    ⁢                                                                                          ⁢                                                                                                                                  N                              FFT                                                                                      K                              act                                                                                                      ⁢                                                  ∑                                                      k                            =                            0                                                                                                              N                              FFT                                                        -                            1                                                                                                                ⁢                                                                                                                                                                                  (                                                                  c                                                                              mL                            +                            l                                                    ,                          k                                                                    ·                                                                        h                                                      l                            ,                            k                                                                          ⁡                                                  (                                                      t                            -                                                          mT                              TPLF                                                        -                                                          T                              AMSS                                                                                )                                                                                      )                                                                        ]                              
The above example shows that any values may be used for N, K, L1 and L2. However, the K data carriers may also be grouped into segments. For purposes of illustration, FIG. 8 shows a graphical representation of a grouping of K data carriers into segments. The K used carriers are divided into six segments 810, 812, 814, 816, 818, 820, as an example. The carriers are plotted along a frequency axis 830, which describes the frequency either directly or via an index of a sub-carrier. Each segment 810, 812, 814, 816, 818, 820 includes 4 carriers 840 in the illustrated example. In the illustrated example, a zero carrier 850 is further respectively put in between the segments 810, 812, 814, 816, 818, 820, wherein the expression “zero carrier” refers to a carrier with the amplitude zero. The zero carriers may thus also be regarded as unused sub-carriers.
Assuming a signal is transmitted that contains various subsignals, for example i broadcast programs, which are combined to form a multiplex, the segments may be associated with certain groups. In the example, four groups are illustrated. In the illustrated example, the first segment 810 and the third segment 814 belong to the first group (group 1), wherein the two segments 810, 814 which the first group (group 1) is composed of are designated “group 1,1” and “group 1,2”, respectively, for a better differentiation. In other words, the last number in the designation of the group consecutively numbers segments associated with the same group.
In the example shown in FIG. 8, four groups are selected which may also have different data rates. The number of segments associated with a group may then be selected correspondingly. In the illustrated example, the first segment 810 and the third segment 814 are associated with the first group, while the fifth segment 818 and the sixth segment 820 are associated with group 4. The second segment 812 is associated with the second group, and the fourth segment 816 is associated with the third group.
In other words, two segments each are associated with groups 1 and 4.
The illustrated segmentation offers various advantages:                Only those segments have to be further processed in a receiver that are associated with the selected group.        The number of transmitted segments may be configured. Thus scaling of the bandwidth is possible.        It is further possible that the selected constellation is different for the segments in the OFDM carriers.        
In the following, satellites having antennas with high directivity, i.e. spot beam antennas (also referred to as spot beam satellites), are described. Most satellites available today are implemented for the coverage of a relatively large area. Thus, a satellite may, for example, be implemented to cover the whole of Europe. Examples of such satellites implemented for large-area coverage are the “ASTRA” satellites or the “AfriStar” satellite. Newer satellites allow a signal to be focused only on smaller regions (also referred to as “spots”). For illustration, FIG. 9 shows an exemplary graphical representation of a spot beam structure. The size of a spot beam and/or a region (i.e. the size of a spot) depends on the frequency range and the properties of the satellite antenna. For the example used, the size of a spot approximately corresponds to the size of countries, such as France, Italy or Germany. The boundaries of the spots and/or the regions indicated as hexagons in FIG. 9 and identified by numbers are not to be regarded as hard boundaries. What is illustrated are rather the boundaries of the region in which the field strength falls below a certain minimum value. Thus, for example, the signal from region “99” (which basically includes Germany) may also be received in large parts of France, which is approximately covered by region 84.
In other words, a region is an area within which a receive signal has a useful field strength greater than a minimum field strength (also referred to as threshold field strength). In adjacent regions, the same signal may still be received, but with a lower field strength.